The long term goals of this research are to identify the interneurons which mediate the primary afferent depolarization (PAD) of gr. I muscle and of cutaneous fibers in the cat spinal cord, and to define more precisely the reflex pathways to which they belong, as well as the supraspinal influences which modulate their activity. Particular attention will be given to determine if the interneurons mediating the PAD of gr. I fibers belong to private pathways exerting presynaptic actions only, or if they are shared also by pathways producing post-synaptic inhibition in motoneurons. I.- In the anesthetized cat, simultaneous extracellular recordings will be made from interneurons in the intermediate nucleus and from dorsal root and ventral root filaments, using in the latter case the sucrose gap technique. Spike triggered averages will be used to enhance any dorsal root and/or ventral root potentials which are time locked to the interneuronal activity. This will allow disclosure of the synaptic connections of the interneurons with afferent fibers and/or motoneurons. Detailed investigation of the responsee patterns of these interneurons to segmental and supraspinal inputs will be also made in order to define the reflex pathways to which they belong. Attempts will be made to record intracellularly from the PAD mediating interneurons and to inject them with horse-radish peroxidase for their morphological characterization. II.- Our previous investigations suggest that there are two independent sets of intermediate nucleus interneurons mediating the PAD of the Ia fibers which are subjected to opposite actions from the brain-stem. since the criteria which we have used to classify gr. I fibers of extensor muscles have been based on the patterns of the PAD produced by stimulation of afferent nerves, these studies will now be extended to gr. I fibers from the posterior biceps-semitendinosus where classification of gr. Ia and Ib fibers is less equivocal. These investigations are expected to provide information pertaining the functional organization of the interneurons which mediate the PAD of the gr. Ia fibers of flexors and extensors. III.- We will study the changes in the mean, variance and correlation of the single fiber Ia monosynaptic EPSPs recorded simultaneously from motoneuron pairs during varying amounts of stretch of agonist and antagonist muscles. Particular attention will be given to the possible existence of correlated failures of the EPSPs generated in the two motoneurons as evidence for conduction blockade in presynaptic terminals.